The invention relates to a process for producing urea fertilizer by means of a synthesis stage, an evaporation unit, and a granulation unit, wherein a self-regulating centrifugal pump is used to reduce the residence time and optimize the positioning of the process components for conveying the product stream from the evaporating operation to the granulation unit.
Processes for producing granulated fertilizers have been extensively described in the technical and patent literature, for which U.S. Pat. No. 6,203,730 B1, DE 28 25 039 B2, and U.S. Pat. No. 4,943,308 A may serve as examples. Typically the product stream, in the form of a melt or solution, arriving from the synthesis stage, is supplied to an evaporator, for the purpose of adjusting the water content, and then passed into the granulation stage. In certain prior-art processes, introduction into the granulator is preceded by mixing with a fine solid fraction, which is generally recycled granules.
With production volumes of 1500×103 kg per day or more, the optimum positioning of the individual process components is a parameter critical to the functionality and efficiency of the process as a whole. In this context it is important that liquid and solid media are as far as possible moved by means of gravity, without additional conveying equipment. The positioning of the granulator, the largest component apart from the synthesis reactor in this process, determines the disposition of the other components, and is typically arranged at a height of 8 m to 15 m. This allows the finished granules to reach the subsequent and lower-lying apparatus without a conveying equipment. Furthermore, for example, the rinsing liquid, which is regularly obtained in large quantities, can be collected via a free discharge directly beneath the granulator.
The product stream leading into the granulation unit is typically conveyed by means of a centrifugal pump, which is arranged a few meters downstream of the evaporation unit. In this arrangement the evaporation unit, by virtue of its elevated disposition, constitutes the barometric endpoint for the centrifugal pump and thus ensures the necessary pump intake pressure. Disadvantages of this positioning are the need for a high platform for a very large and heavy component and the associated piping, which unfavorably prolong the residence time of the product stream. Furthermore, these lines must be given costly and sophisticated insulation and tracers, in order to prevent incrustations on the pipe walls or of separation, and to maintain the high temperature until the final drying operation is performed in the granulator.
It is known from DE 197 44 404 that, as a function of the temperature and of the residence time in the product solution, secondary reactions of the urea occur, forming essentially polymers and condensates of urea, which have no biogenic effect and therefore lower the concentration of active compound in the granules. As well as these inactive secondary products, biuret is formed as well, which has a strongly phytotoxic effect and must therefore be suppressed as far as possible.
The concentration of the highly phytotoxic biuret in the urea solution actually increases as the temperature and residence time go up. The relationships affecting biuret formation are described in, for example, AT 285621, CH 617 672A or GB 1 404 098, and thus have been known for more than 30 years.
In the evaporation unit it is necessary to set a temperature of 110° C. to 150° C. in order to remove the required amount of water from the product solution within an adequate time. As a result, this high temperature is likewise present in the downstream pipelines, and this greatly promotes the formation of biuret. Accordingly, in the pipelines downstream of the evaporation unit, there is biuret formation to an extent which affects the end quality of the product, and its formation may be additionally intensified by further overheating of these pipelines, which are generally heated using steam.
DE 197 44 404 discloses a process in which the addition of dicyandiamide produces a crystallization inhibition effect, so that the process can be operated at temperatures of 70-90° C., thereby sharply reducing the formation of biuret. A disadvantage of that process is the high consumption of dicyandiamide, which is biologically inactive and which according to the example given in DE 197 44 404 must be added at more than 5% by weight per hour. This addition of an additive necessitates a correspondingly larger design volume and considerably increases the operating costs of the process.
A further disadvantage is that the endothermic dissolution of the dicyandiamide consumes energy, which must be supplied again at least partly to implement the subsequent drying operation in the granulator.
An important source of the biuret formation is the evaporator, and so JP 57171956 A proposes a specific operation of concentrating the solution, by designing the evaporator as a spraying tower, so that the large liquid surface area available in the tower permits a very rapid and efficient removal of water from the product solution. A disadvantage of a solution of this kind is the significantly more sophisticated evaporator design, which additionally necessitates significantly more complex control and instrumentation devices.
GB 1 404 098 proposes a process in which the urea solution prior to granulation is passed over an ion exchanger in order to separate the biuret. That process necessitates additional process components and causes higher operating costs as a result of the need to regenerate the ion exchanger material at regular intervals.